Read-through cache callbacks

Read-through cache callbacks are invoked when an item cannot be retrieved
from the server. The callback is passed the Memcached object, the requested
key, and the by-reference value variable. The callback is responsible for
setting the value and returning true or false. If the callback returns true,
Memcached will store the populated value on the server and return it to the
original calling function. Only Memcached::get() and
Memcached::getByKey() support these callbacks,
because the memcache protocol does not provide information on which keys
were not found in the multi-key request.